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The NIF achieved what is known as scientific breakeven, defined as the fusion energy released from a plasma exceeding the energy that was used to create and heat the plasma. The process of inertial fusion involves shooting a fusion “target” in the center of a large vacuum chamber with a high-energy short laser pulse. The laser energy is ultimately used to compress, heat, and ignite fusion fuel in the target, and the accepted definition of scientific breakeven for inertial fusion is when the fusion burst from the target exceeds the energy of the laser pulse that fires into the chamber toward the target.

To date, the NIF is the only facility that has achieved scientific breakeven in the lab, and the current highest performing shot at the NIF produced 5 MJ of fusion energy from a 2 MJ laser pulse, corresponding to a “scientific gain” of 2.5. To date, the next-best performing system is the JET tokamak, which achieved a “scientific gain” of 0.67.

To answer this we need to briefly describe a NIF fusion target. A NIF target consists of a heavy metal (such as gold) can about 1 cm in size with a 2 mm diameter spherical fuel capsule situated inside the can. The fuel capsule contains roughly 0.2 milligrams of fusion fuel, as well as a plastic layer on the outside surrounding the fuel. On the NIF, the laser beams do not hit the fuel capsule directly, but instead hit the inside of the can. This creates an intense bath of x-rays that fills the can and heats the plastic layer on the fuel capsule. The plastic layer blows off in billionths of a second creating a compressive force inwards that implodes, heats, and ignites the fusion fuel inside. While the NIF laser delivers 2 MJs of energy to the can, only 250 kJ of energy is actually absorbed by the fuel capsule from the x-rays. The implosion of the capsule can be thought of as a spherical "rocket" with the fusion fuel as the payload, and like a rocket’s efficiency only about 10% of the absorbed energy (~25kJ) ends up in the compressed fuel as heat.

What is significant is that the fuel capsule itself produced 5 MJ of fusion energy when only absorbing 250 kJ of energy. This corresponds to a fuel capsule gain of 20! This gain was possible because a fully ignited and self-sustained burning plasma was produced. This is highly scalable, and up to 10x higher fuel capsule gains can be achieved by imploding a much larger capsule.

Let’s take a look at the overall energy budget of NIF’s highest performing shot to date. 400 MJ were taken from the grid and stored in capacitors to power the laser. The laser produced 2 MJ of optical energy delivered to a NIF “target”, which consists of a fuel capsule about 2 mm in size inside a heavy metal can about 1 cm in size. The laser pulse was delivered to the inside wall of the can, which was heated by the laser to millions of degrees, creating a thermal x-ray bath inside the can. The fuel capsule absorbed roughly 250 kJ of energy from this x-ray bath, causing it to implode, ignite, and burn, releasing 5 MJ of fusion energy. Thus the fuel capsule gain was 20, the target gain (equivalent to the scientific gain) was 2.5, the fuel capsule absorbed 12% of the energy that was in the laser pulse, and the laser efficiency itself was 0.5%. This corresponds to an overall “wall-plug” energy gain of just over 0.01, or 1%.

A commercial system must have a wall-plug gain of ~10, as opposed to the 1% achieved on the NIF. This might seem like a major gap, but with a much more efficient and energetic laser, it is not as challenging as it might seem. It is important to note that NIF was never designed for efficiency and the laser is based upon technology of the 1990’s. The NIF was built for science to support the national security mission of the National Nuclear Security Administration (NNSA) to help ensure the safety and reliability of the US nuclear deterrent.

In an Xcimer system, we will achieve 10x higher fuel capsule gain by absorbing over 30x more energy into a much larger capsule, we will achieve over 10x higher laser efficiency through the use of excimer lasers, and we’ll couple over 90% of the laser energy directly to the fuel capsule, vs. only 12% coupled via the x-ray bath on the NIF. These together provide a 1000x increase in wall-plug gain compared to the NIF, allowing for a commercially viable system.

While the NIF has made an amazing achievement, there are several significant challenges in making the laser system commercially viable.

• The laser technology is far too expensive to scale to 10 MJ, which is the energy level needed for robust fusion target operation. The NIF contains over 120 tons of high quality laser glass, as well as over 4000 square meters of optical surfaces to direct the laser beams to a target. This large volume of precision optical material contributed a significant amount to NIF’s $3.5B cost.
• The NIF can only shoot a few times a day, vs a shot every few seconds. Even at 2 MJs of energy, the NIF experiences a significant amount of damage to its optics on every shot - over $40M per year is spent on refurbishment of the optics. Going to 1 shot every few seconds would simply be unaffordable.
• The NIF has 192 beams and 30 square meters of final optical area. This high number of penetrations and large final optical area exposed to the target make it infeasible to use a coolant as a liquid wall to protect the reactor chamber.

While there have been significant advancements in solid-state laser technology since the NIF was built, solid-state glass systems are still very expensive, not scalable to 10 MJ, and must still use a large number of laser beams to hit a target.